1. Field of the Invention
This invention relates to a light beam deflector, and more particularly to a light beam deflector that uses an acousto-optical element to deflect the light from a predetermined light source.
2. Description of the Prior Art
As the light deflection means in, for example, laser-beam scanning microscopes and laser-beam scanning video recorders, it is known to use of an arrangement employing rotating mirrors or acousto-optic elements. Deflectors utilizing acousto-optic elements are particularly advantageous in that they have no mechanical moving parts and are stable and longlasting, and enable high scanning frequencies to be realized.
The structure of a known light beam deflector employing an acousto-optic element is shown in FIG. 6. In FIG. 6, light from a laser light source or the like is projected onto an acousto-optic element 1 as indicated by the symbol 2, and is deflected by the acousto-optic element 1. An aperture 5 in the drawing is for the purpose of obtaining only first-order diffraction light.
With this arrangement, by changing the frequency f of a high-frequency drive signal applied to the acousto-optic element 1, the angle .theta. of the first order diffraction light isolated from the incident beam (usually laser light) can be changed. The high-frequency drive signal is generated by a voltage controlled oscillator and is powered by the broadband amplifier 8.
However, there is a problem in that as the intensity (deflection efficiency) of the first-order diffraction light of the incident beam is not constant with respect to the drive frequency f, owing to the particular properties of the acousto-optical element. Accordingly the light intensity is dependent on the angle of deflection .theta..
FIG. 7 shows an example of the correspondence of deflection efficiency characteristics to the drive frequency. The characteristic prior art curve of FIG. 7 has a peak in the medium-high zone and in the medium-low zone.
In resolving this problem, a known technique has been used heretofore which is based upon the characteristic that the deflection efficiency being is substantially proportional to the intensity of the element drive signal. A waveform that approximates a deflection efficiency characteristic that is the inverse of that shown in FIG. 7 is produced by an analog type function generator circuit. By using this signal for modulating the amplitude of the element drive signal, nonuniformity in the deflection efficiency is cancelled out by changes in the drive power.
However, the deflection efficiency characteristics are affected not only by the properties of the acousto-optic element itself, but also by the frequency characteristics of the overall drive circuitry and the linear characteristics of the amplitude modulation circuitry. This being the case, it is extremely difficult to produce a perfectly inverse characteristic waveform by means of an analog type function generator, taking into account the overall characteristics of these circuits.
In addition, deflection efficiency is also dependent on the wavelength of the incident light beam, the angle of incidence of the light, the scanning frequency (repetition frequency of the deflection) and the like, and with the conventional system it has not been easy to adapt in cases where all of these conditions have changed.
On the other hand, there is a known technique in which a portion of light deflected by the acousto-optic element is detected by a photosensor and photoelectrically converted into a signal. The thus produced signal is applied to the driving circuit as a direct feedback to control the uniformity of the light intensity.
However, the deflectors utilizing the acousto-optical effect define an inherent time (access time), generally more than about several micro-second, which is required for supersonic waves traveling in the acousto-optical medium to traverse the light beam. As a result, the feedback system suffers from such lag time and the speed of the control becomes inevitably low. In such a device, it is thus impossible to correct completely the nonuniformity of the deflection efficiency in the high frequency scanning system.
There has therefore been a problem in that where a light beam deflector employing an acousto-optic element is used for video image input/output operations, such as for example in a laser-beam scanning microscope or laser-beam scanning video recorder, with a conventional system, with respect to the direction of deflection by the element, there remain variations in light intensity which cannot be fully corrected.